If a ship drops that you already have, you get another one from the list

If you have all the ships and you drop a ship AND you already have all the ships possible, you will get the doubloon value, port slot and a 10 point commander.

WG, in theory has fixed the drop mechanic that gave multiples of the same ship in the same purchase. This likely means that the doubloon value ships are going to be the original "Brotherhood of Santa Claus" ships that are mentioned not the ones that are possible. So selling you the highest value ship and then buying more boxes likely isn't going to get you as many doubloons as last year.

Also unlike last year where you could specify the number of crates you wanted to buy, this year they're available in 1x 5x and 20x bundles only. The 5x and 20x bundles are discounted between 11% and 25% depending on the Captain/Commodore/Admiral boxes.

Some people will buy a few crates or bundles and get a ton of ships. On the flip side some will dump a ton of cash and get very little or nada ships and a ton of flags and camo.

Reading the Santa crate threads on reddit covering RU and NA purchases, it still a bit hard to tell if they have changed. If anything it looks like the drop rates have improved on the large crate vs the medium one, vs last year where it was the opposite.